<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h3>
    结果:<span></span>
</h3>
<script>
    let a=document.querySelector("#i1")
    let b=document.querySelector("#i2")
    let s=document.querySelector("span")
    function f(){
        if (isNaN(a.value) || isNaN(b.value)) {
            s.innerText = "输入错误！";
            return true;
        }else {
            return false;
        }
    }
    function f1() {
        if(f()){
            return;
        }
        s.innerText=a.value*1+b.value*1;
    }
    function f2() {
        if(f()){
            return;
        }
        s.innerText=a.value-b.value;
    }
    function f3() {
        if(f()){
            return;
        }
        s.innerText=a.value*b.value;
    }
    function f4() {
        if(f()){
            return;
        }
        s.innerText=a.value/b.value;
    }
</script>
</body>
</html>